1. Field of the Invention
The present invention relates in general to a controller and a control method for orbit boring, and in particular, to a controller for and a control method of controlling an acceleration in an orbit boring.
2. Description of the Related Art
There has been developed a turning operation method for NC (Numerical Control) machine tools (Japanese Patent No. 3093935), in which a boring bar (tool) mounted on a spindle is controlled to always be oriented in a radial direction (normal direction) of circular arc, by the synchronization of a circular interpolatory movement of a center of the spindle on a plane perpendicular to the spindle and a rotational movement of the spindle, to execute a boring process.
This turning operation method, called orbit boring from the mode of movement, is advantageous in that the boring of an arbitrary hole size can be performed with a single boring bar.
The machining precision (circularity) of orbit boring receives influences such as from an elliptical error or radius reduction error of a spindle or feed members elongated by inertial forces and a pulsatory error due to vibrations of an associated structure in the tooling machine. Such errors increase, as the acceleration increase in the direction of feed. FIG. 8 shows an experimentally observed relationship the circularity (C1 in xcexcm) or inertial-force derived error (C2 in xcexcm) has to the acceleration (G). As is apparent, increase in acceleration causes an increased error of circularity (C1) in terns of a radial difference between imaginary circles inscribing and circumscribing a machined circular configuration,
This error in circularity is derived from accelerations in orbit boring. It is observed that the machining precision or accuracy (circularity) in the orbit boring is degraded substantially in proportion to an acceleration in the feed direction, with a problem to be experienced in a high-velocity or high-acceleration orbit boring. In actual, the velocity and viscous friction also have their influences. However, the inertial force as well as vibrating force has an increased magnitude in proportion to the second power of the velocity, and occupies the most part of cause of the degradation of circularity, providing the base of a conclusion that the error in circularity be essentially derived from the acceleration.
In cases where the radius of machining by orbit boring changes, such as in a taper machining, the acceleration in the feed direction varies in inverse proportion to the machining radius, so that the machining precision of orbit boring is varied with a varying radius of machining such as in taper machining, with a failure to achieve a conforming circularity.
The present invention has been made with such points in view. It therefore is an object of the invention to provide a controller for and a control method of controlling an acceleration in an orbit boring to suppress or limit a machining error due to the acceleration, so that the machining precision resides within a permissible range or meets requirements therefor.
To achieve the object, an aspect of the invention provides a control method for an orbit boring to be performed by a tool in an orbital motion along an orbit, comprising providing a permissible range of an acceleration of the orbital motion to meet a criterion of a machining precision of the orbit boring, and controlling the acceleration to be suppressed within the permissible range.
Further, to achieve the object, another aspect of the invention provides a controller for an orbit boring to be performed by a tool in an orbital motion along an orbit, comprising a provider for providing a permissible range of an acceleration of the orbital motion to meet a criterion of a machining precision of the orbit boring, and a controller for controlling the acceleration to be suppressed within the permissible range.